1. Field of the Invention
The present invention relates to watermarking, and more particularly, to a digital image watermarking method.
2. Description of the Related Art
Digital watermarking is a technique of embedding a bit pattern, which is referred to as signature information, in a digital image or audio and video files to identify the image or files as copyright information. That is to say, digital watermarking can be used for the purpose of providing copyright protection. In the case where copyright material is based on video contents, the digital watermarking is devised such that the signature information is invisible. In the case where copyright material is based on audio contents, the digital watermarking is devised such that the signature information is inaudible. Also, the signature information must be evenly spread throughout an original file so as not to be identified or manipulated by users. Further, the signature information must be robust enough to withstand general changes made to the original file, for example, reduction due to a compression algorithm.
FIG. 1 illustrates a conventional digital image watermarking method. Referring to FIG. 1, in the conventional digital image watermarking method, a host image and a signature image are transformed into Fourier coefficients by fast fourier transform (FFT), and not only coefficients of the transformed signature image but also a password or key for mixing are attached to a high frequency portion of a coefficient spatial domain of the transformed host image, thereby performing watermarking. Now, the watermarked image is separated into host image coefficients and signature image coefficients using a password or key for separation. Next, the separated coefficients are inverse fast fourier transformed (IFFT), thereby obtaining a host image and a signature image, respectively.
Also, according to another conventional digital watermarking method, a signature image and a host image are transformed into wavelet coefficients by discrete wavelet transform (DWT), and the wavelet coefficients of the signature image are attached to a wavelet spatial domain of the transformed host image, thereby performing watermarking.
However, according to the conventional watermarking methods, since the arrangement of signature image coefficients embedded in the host image coefficients is based on the arithmetic sum of two coefficients at each pixel point, the arrangement robustness cannot be ensured. That is to say, if a combined image undergoes severe image processing for altering the image, e.g., image compression, the arithmetic sum may be changed so severely that one type of data cannot be separated from another type of data. Similarly, if a combined image undergoes mild image processing two or more times, the arithmetic sum may also be changed so severely that one type of data cannot be separated from another type of data.